OBJECTIVES: (1) Long term reactions of visual pigments to light, darkness and hormones. (2) Biochemical and photobiochemical relationships between the pigment epithelium and retinal photoreceptors. Particular regard is given to the interchange and storage of retinols, their esters and their isomers between these structures during light and darkness, and the role of the pigment epithelium in visual pigment regeneration and biosynthesis. The role of the pigment epithelium in the degradation and disposal of shed packets of ROS discs (phagosomes) will also be investigated. (3) The enzymes and retinol binding proteins involved in the transport and metabolism of vitamin A during bleaching and regeneration will be investigated. (4) The evolution of visual systems will be studied, with particular emphasis on developments resulting from isolation of fish species and populations through recent glacial action. This study will lead to a better understanding of the structure of visual pigment proteins (opsins) and the enzymes associated with the synthesis of visual pigment prosthetic groups (retinal and dehydroretinal).